Changes in rhizosphere bacterial communities associated with tree decline: grapevine esca syndrome case study.

An investigation was carried out on rhizosphere bacteria to determine if they may be associated with perennial crops affected by nonspecific decline, a phenomenon that is difficult to diagnose and prevent. Esca disease of grapevine was chosen for this case study because of its easy foliar symptom identification. Ribosomal DNA fingerprint analysis by polymerase chain reaction - denaturing gradient gel electrophoresis (PCR-DGGE), quantitative PCR (qPCR), and rDNA amplicon sequencing by next-generation sequencing (NGS) were adopted to investigate the bacterial communities associated with grapevines, which were selected for the presence and absence of external foliar symptoms in 11 vineyards. According to PCR-DGGE and qPCR, bacterial communities differed in site of origin (vineyards), but not between symptomatic and asymptomatic plants, whereas qPCR gave a significantly higher presence of total bacteria and Pseudomonas spp. in asymptomatic plants. NGS confirmed no difference between symptomatic and asymptomatic plants, apart from a few minor genera (<0.5%) such as Salinibacterium, Flavobacterium, Nocardia, and Janthinobacterium, which were, in all cases, higher in asymptomatic plants and whose functional role should be the object of further investigation. The fact that total bacteria and Pseudomonas were more abundant in the rhizosphere of asymptomatic grapevines and that some bacterial genera were associated with the latter, represents a new element when investigating the multiple-origin phenomenon such as esca disease of grapevine.